This invention relates to a silicon wafer (hereinafter sometimes simply referred to as the “wafer”) suitable for use as a substrate on which a semiconductor device is to be manufactured, and a method for manufacturing such a wafer.
For a silicon wafer for use as a substrate on which a semiconductor device is to be manufactured, it is required that its surface layer which is used as an active region where a semiconductor device is to be manufactured be free of defects by reducing e.g. crystal-originated particles (COPs) and laser scattering tomography defects (LSTDs).
In order to improve productivity of such a silicon wafer, in a recently developed method, at least the surface of the silicon wafer on which a semiconductor device is to be manufactured is mirror-polished, and then the silicon wafer is subjected to a rapid thermal process (hereinafter abbreviated to “RTP”).
For example, JP Patent Publication 2001-509319A discloses a thermal treatment in which the wafer is heated at a temperature exceeding 1175° C. for less than 60 seconds in an argon or helium atmosphere containing oxygen under the oxygen partial pressure of less than 5000 ppma.
The heat treatment disclosed in this patent publication can markedly reduce COPs on the surface layer of the wafer because RTP is performed in an inert gas atmosphere of which argon or helium is the major component.
However, during RTP in an inert gas atmosphere, oxygen in the surface layer of the wafer tends to be dispersed outwardly, so that the oxygen concentration in the surface layer decreases. This reduces the pinning effect of oxygen during heat treatment in the later step of manufacturing a semiconductor device. Thus, the higher the temperature of the heat treatment, the more likely slip dislocations are to occur.
In order to avoid this problem, JP Patent Publication 2010-129918A proposes to subject a semiconductor wafer to a heat treatment of not less than 1000° C. and not more than the melting point in a furnace in an oxygen-containing gas atmosphere, thereby inwardly dispersing and introducing oxygen into the surface layer of the wafer, and then take the wafer out of the furnace to fix the high solid solubility of oxygen in the surface layer of the wafer. By performing this method, it is possible to increase the oxygen concentration in the surface layer, thereby increasing the strength of the surface layer.
However, if RTP is performed in an oxygen-containing gas atmosphere as disclosed in JP Patent Publication 2010-129918, especially if the RTP is performed at a temperature of less than 1300° C., there are the following two problems. One is that since the oxygen concentration in the surface layer of the wafer has increased due to inward dispersion of oxygen, oxide films on the inner walls of COPs are less likely to melt, so that COPs cannot be easily removed. Another problem is that since it is impossible to melt oxygen precipitation nuclei produced while crystals are growing, if the nuclei have ununiform density and size distributions in the radial direction of the surface of the wafer, these nuclei could grow into oxygen precipitates that are ununiform in density and size during the later heat treatment step, thus reducing the strength of the wafer.
On the other hand, if the heat treatment temperature is 1300° C. or higher, it is possible to melt oxide films on the inner walls of COPs, thereby removing the COPs, and also melt the oxygen precipitation nuclei produced while crystals are growing. In this case, however, oxygen precipitation nuclei tend to be produced in high density in the surface layer of the wafer as the active region where the semiconductor device is to be manufactured.